Benzo(a)pyrene (BP) is metabolized either to nontoxic water soluble conjugates or to the ultimate carcinogen, BP-7,8-diol-9,10-epoxide. The direction of BP metabolism might be dependent on the presence of specific predominant forms of enzymes involved in the BP metabolism of individuals exposed to chemical carcinogens. Monoclonal antibodies (MABs) are specific to antigenic determinants and are useful tools to test and locate specific enzymes. Cytochrome P-450 and epoxide hydrolase (EH) are key components of the system responsible for the metabolism of BP. In order to prepare MAB probes to these enzymes Balb/c female mice were immunized with pure preparations of cytochrome P-450 and human epoxide hydrolase. The primed spleen cells were fused by polyethylene glycol with myeloma cells and hybridomas were screened for antibody produced against cytochrome P-450 and EH. Four hybridomas produced MABs which bind to human liver cytochrome P-450 but do not cause precipitation. Against EH, four hybridomas produced MABs which bound and precipitated only the form which was used as antigen but not other forms. Effects of the MABs on enzyme activities and the cellular location of the antigens are yet to be clarified.